The short answer
Partial response, stable disease and progression are measurement categories from a scan rulebook called RECIST. Stable disease means the treatment is holding the line, not failing.
These terms measure tumour size on scans using a standard rulebook (RECIST 1.1), not how well you are doing overall.
Partial response means the sum of the measured tumour diameters has fallen by at least 30% from baseline.
Progressive disease means a rise of at least 20% from the smallest sum recorded, with a minimum absolute increase of 5 mm, or any new lesion.
Stable disease is everything in between — a tumour that shrank 25% and one that grew 15% both fall into it.
Choose how you want to understand this
The full explanation.
What these words are measuring
Partial response and stable disease are not descriptions of how you are doing. They are measurements of tumour size on scans, made using a standard rulebook so that results mean the same thing in different hospitals and different trials. For solid tumours, that rulebook is usually RECIST, currently version 1.1.
Understanding that these are measurement categories — not grades, not verdicts, not predictions — takes a surprising amount of weight off them.
The baseline scan
Before treatment starts, a radiologist selects a small number of measurable tumours, called target lesions, and measures the longest diameter of each. Those measurements are added into a single number: the sum of diameters. That sum is the baseline.
Every scan afterwards measures the same lesions and produces a new sum. Response is judged against the baseline; once the tumour has shrunk, progression is judged against the smallest sum recorded so far, called the nadir. This is why the same lesion size can be described differently at different times, and why comparing your latest scan only with your previous one can mislead.
The four categories in plain words
- Complete response: all target lesions have disappeared.
- Partial response: the sum of diameters has fallen by at least 30% from baseline.
- Progressive disease: the sum has risen by at least 20% from the smallest it has been, and by at least 5 mm in absolute terms — or a new lesion has appeared anywhere.
- Stable disease: neither of the above. Not enough shrinkage to call a partial response, not enough growth to call progression.
The 5 mm rule exists for a reason worth knowing. Measurement is not perfectly precise. A 20% change in a small lesion can be a couple of millimetres, well within the ordinary variation between radiologists, scanners, and how you happened to be breathing. The absolute minimum stops noise being reported as progression.
Notice how wide stable disease is. A tumour that shrank 25% is stable disease. One that grew 15% is also stable disease.
Stable disease is not treatment failure
This is the most common misreading of these reports. Stable disease means the treatment is holding the line. For many cancers and many treatments — particularly in advanced disease — holding the line is precisely the goal, and prolonged stable disease can mean a long period of good control and good quality of life.
Response rate and benefit are not the same thing. Some effective treatments shrink tumours; others mainly stop them growing. A report that says stable disease usually translates as: stay on this, come back in a few weeks.
Where the categories get blurry
- Some disease is not measurable — bone metastases, a thin layer of tumour, fluid in the abdomen. These are tracked as non-target disease and described in words rather than numbers.
- With immunotherapy, some tumours appear to grow before they shrink as immune cells flood in. A modified set of criteria, iRECIST, exists so that a working treatment is not abandoned for this reason, which is why your team may repeat a scan rather than act on a single result.
- Different radiologists may choose slightly different lesions or judge edges differently. Small changes between reports are common and often mean nothing.
- Tumour markers in the blood do not always move in step with the scans, and neither one alone is the whole picture.
- RECIST was designed to compare treatments across groups of people in trials. It was never intended as a verdict on one person's situation, which is why your oncologist reads it alongside your symptoms, examination and bloods.
What to ask when you get a result
Ask what your scan was compared against, and by how many millimetres things actually changed rather than which label was applied. Ask whether the change sits inside the range of ordinary measurement variation. Ask whether the plan changes as a result, and if not, what would need to happen for it to change.
And ask what stable means for your specific situation — because in the middle of treatment for advanced cancer, it is frequently good news wearing a bland word.
Sources
Words to know
Tap any term to see what it means.
Common questions
My scan says stable disease. Is the treatment working?
Usually yes, in the sense that matters. Stable disease means the tumour has neither shrunk enough to count as a response nor grown enough to count as progression. For many advanced cancers, keeping disease from growing is precisely the aim, and prolonged stable disease can mean many months or years of good control.
Why does a few millimetres of growth not count as progression?
Because measurement is not perfectly precise. A 20% change in a small lesion can be one or two millimetres, which is within normal variation between radiologists, scanners and how you were breathing. The rule requires both a 20% relative increase and at least 5 mm absolute, so that noise is not reported as progression.
My last scan was compared to something different. Why?
Response is judged against the original baseline, and progression against the smallest sum recorded during treatment (the nadir). This is deliberate: it stops a tumour that shrank a lot and then regrew slightly from being scored as a continuing response.
What if my tumours grew at first on immunotherapy?
Some tumours appear to enlarge before shrinking as immune cells move in. A modified rulebook, iRECIST, exists so that a working treatment is not stopped for this reason, which is why your team may repeat a scan in a few weeks rather than act on one result.
Why doesn't my tumour marker match my scan?
Blood markers and imaging measure different things and do not always move together. Neither on its own is the whole picture, and your team interprets them alongside how you feel and your examination.
Questions to ask your doctor
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Written by: Cancer Explained Editorial TeamSources last checked: 2026-07-30Last updated: 2026-07-30Next planned review: 2028-07-29
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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